U.S. Pat. Nos. 5,055,186 and 5,593,574 granted Oct. 8, 1991 and Jan. 14, 1997, respectively, to VanToever, relate to bioreactor systems, primarily biofilter system, using fluidized pellet media Although such systems are effective, efforts to scale up the systems have encountered some difficulties, particularly when the objective is to provide a bioreactor system having maximum possible effective surface area for cultural bacteria and other microorganisms to provide a system which is self cleaning and relatively maintenance free as much as possible and to provide a system which operates with low energy consumption.
More particularly, the revolving downflow injector design described in U.S. Pat. No. 5,593,574 works adequately with shallow filter media beds. The concept of fluidizing only a narrow zone of media at any given time rather than the conventional method of continually fluidizing the entire bed of media enabled a drastic decrease in the energy required for fluidization
Nevertheless, efforts to scale-up such downflow injector filters with greater tank diameters, (&gt;1 m) and media bed depths, (&gt;1 m), using this design, required significant increases in pump size to provide sufficient energy to fluidize the pellets. Since the low density plastic pelleted media is buoyant, (specific gravity of 0.91-0.93 relative to water), the downward directed jets of filtrate must have sufficient force to counter the buoyancy and flotation of the media in order to fluidize the bed. With increased bed depth, the energy required increased significantly. By increasing the pressure and flow of filtrate, deep beds could be fluidized but at exceedingly high, if not, prohibitive operating costs.
Additionally, the increased turbulence caused by the high energy injection would frequently cause media pellets to wash out of the filter.
Extensive efforts have lead to the development of a new, much superior configuration.
Initially efforts focused on slowing the rotation of the downflow filtrate injector system represented by U.S. Pat. No. 5,593,574. The fluidization of a given zone is not instantaneous and a period of time is required for the jets of filtrate to penetrate and fluidize a given cross section of media. Efforts to improve the system included the use of low speed gear motors to slow and accurately control the speed of rotation to ensure complete fluidization. With larger beds, that is, with media beds greater than 1 meter in diameter, rotational speeds of 1/4 rpm and filtrate flows of approximately 600/1/min/m.sup.2 of filter bed surface area were required. However, faster rotational speeds tended to result in incomplete fluidization of the media Further, in order for downward directed jets of filtrate to fluidize the media, the jets had to have sufficient energy to counteract the upward flotation, (buoyance), of the media as well as to counteract the friction in the media bed.
Accordingly, it would be advantageous to provide an injector system to fluidize the media bed which would ensure that all areas of the filter media bed receive as uniform a flow of filtrate as possible and which could be expanded radially or indepth to encompass larger media beds.
The earlier U.S. Pat. Nos. 5,055,186 and 5,593,574 referred to above, utilize plastic media pellets and the system to which this invention is directed also depends on the use of plastic media pellets. The purpose of the media is to provide an optimal `engineered` surface area for culturing bacteria, fungi and other microorganisms, while at the same time providing the maximum possible effective surface area per unit volume of filter at a reasonable cost. The desired microorganisms require a surface to colonize and with the appropriate nutrients and environment a diverse ecological mix of species establishes and grows to create a biofilm. The biofilm adheres to the substrate--media pellets--and will generally flourish and grow until it plugs the interstitial spaces between the supporting media and blocks the flow of nutrients to the microorganisms. Additionally, particulates in the filtrate also adhere to the "sticky" biofilm through a number of mechanisms and serve to accelerate the plugging of the filter. An effective filter therefore has to continually harvest excess biofilm and particulates in order to maintain an optimal biofilm which is constantly in a growth phase condition, rather than one that cycles between "start-up-growth-plugging- crashing-cleaning-start-up". The fluidized bed design can provide an environment wherein excess biofilm is continually scoured off the media, while sufficient shelter is provided to provide an adequate environment for maintenance of a continually self renewing, optimally, thin biofilm.
Conventional fluidized beds generally utilize randomly configured support media such as sand and plastic material. Creased or grooved media pellets are disclosed in the above-noted U.S. patents. Nevertheless, it would be advantageous to have media pellets which have very specific characteristics and which are manufactured to a specific engineered design to optimize film growth and to be compatible with the radial flow injection system developed.
The filter design relies on the buoyancy of the media pellets to maintain the media bed within the filter. Insufficient buoyancy or excessively high filtrate flow rates which result in excess downflow velocities will wash the media out of the filter outlet. Earlier attempts to screen the outlets of the filters proved futile since the biofilm grows rapidly and plugs the screens.
Biofilms for example, have a specific gravity of approximately 1.07 relative to water. The low density plastic pellet has a selected specific gravity in the range of 0.91 to 0.93 so that it floats in water. The media pellet must therefore be designed with sufficient mass so that the ratio of the maximum supportable biofilm mass, to the pellet mass remains less than one or the pellets will sink.
An apparently obvious solution would be to decrease the density of the plastic and increase the buoyancy. A small increase in buoyancy, however leads to drastic increases in the energy required to fluidize the media, especially in the start-up phase when there is no biofilm present to counter the buoyancy of the pellets. Since energy consumption is a critical factor in determining the success of the bioreactor design, significant increases in buoyancy of the media pellets is not a cost effective option.
All characteristics of the pellet must be considered together to achieve a successful design. A balance must be achieved between the cost of materials and manufacturing, the effective surface area for biofilm culture per unit volume of filter and the dimensions of the sheltered grooves which determines the biofilm biomass relative to the mass of plastic per pellet as this relationship determines pellet buoyancy once the biofilm is established. The design of the pellets must be such to minimize interlocking of pellets which increases energy requirements for fluidization. Further, the pellets must be as small as possible to minimize surface area per unit volume while providing adequate mass for buoyancy as described.
Accordingly, it would be advantageous to have pellet media which have proven to be an acceptable compromise between the various design parameters noted above, particularly in fluidized bed systems as set forth herein.